libstdc++: Remove redundant noexcept-specifier on definitions

These destructors are noexcept anyway. I removed the redundant noexcept
from the error_category destructor's declaration in r0-123475, but
didn't remove it from the defaulted definition in system_error.cc. That
causes warnings if the library is built with Clang.

This removes the redundant noexcept from ~error_category and
~system_error and adds tests to ensure they really are noexcept.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* src/c++11/system_error.cc (error_category::~error_category()):
	Remove noexcept-specifier.
	(system_error::~system_error()): Likewise.
	* testsuite/19_diagnostics/error_category/noexcept.cc: New test.
	* testsuite/19_diagnostics/system_error/noexcept.cc: New test.